1. Field of the Invention
The present invention relates to a clamping device for holding a workpiece at a predetermined position during a machining operation of the workpiece.
2. Discussion of the Prior Art
Holding a workpiece at a predetermined position with a predetermined clamping force is important for raising machining accuracy of the workpiece. Many kinds of clamping devices are used according to shapes of workpieces and machining methods. A clamping device generally used has a hydraulic cylinder by which a clamper is moved so as to hold the workpiece at a predetermined position. Since the pressure of oil used for moving the clamper is set to a constant level, it is impossible to select a suitable clamping force according to the shape of a workpiece and the machining method, thereby the machining accuracy of the workpiece tends to decrease. Further, in the event that many kinds of workpieces are machined, it is difficult to properly hold all kinds of the workpieces because each kind of workpiece requires a different clamping force.
To solve the above-mentioned problems, a clamping device having accumulators and a switching valve has been proposed as is disclosed in Japanese Laid-Open Patent Publication No. 63-84833. Oils having different pressures are filled in the respective accumulators. When an operator or a command from a numerical controller operates the switching valve, oil from a selected accumulator is supplied to the cylinder for advancing the clamper. Therefore, the clamping force of the clamper can be changed according to the shape of a workpiece and the machining method.
However, even in the above-described construction, the clamping force of the clamper is merely changed to one of two different clamping forces. It is therefore difficult to hold the workpiece with a suitable clamping force. Further, there is a problem that a plurality of accumulators, pipes and the switching valve require a large space. Furthermore, the use of oil needs a pump which would generate noises and heat.